1. Field of the Invention
The present invention relates to a method of encoding and decoding optical pulse signals in optical code division multiplex transmission, and to a device to realize this method.
2. Description of Related Art
In recent years communication demands have expanded rapidly with the spread of the Internet and other developments. In order to cope with this, high-speed large-capacity networks employing optical fibers and similar are being prepared. In order to construct such high-speed and large-capacity optical networks, wavelength division multiplexing (WDM) transmission methods are indispensable. In particular, so-called dense wavelength division multiplexing (DWDM) methods, in which the wavelength intervals of optical carrier waves assigned to different channels are narrowed and dense wavelength multiplexing on the wavelength axis is employed, are attracting attention.
However, in a DWDM system which realizes this method, because the wavelength bandwidths of usable optical carrier waves are finite, there is a limit to the multiplexing density of a DWDM system. The wavelength bandwidths of optical carrier waves are limited by the fact that the oscillation wavelength band of the semiconductor laser which is the light source generating the optical carrier wave is limited, and by the fact that the wavelength bands which can be transmitted by the optical fiber which is the transmission path are limited. Also, when the wavelength intervals of the optical carrier waves allocated among channels are narrowed, overlap of the optical spectra between adjacent channels (also called “crosstalk”) may cause the problem of degradation of transmitted optical pulse signals.
Transmission using optical code division multiplexing (OCDM) is therefore attracting attention as means of resolving the above problems. In OCDM transmission, optical pulse signals (resulting from optical modulation of optical pulse trains, or conversion of electrical pulse signals into optical pulse signals) for a plurality of channels are generated in parallel, these are modulated by different codes for each channel (encoded), and by decoding on the receiving side using the same code as was used for encoding on the transmission side, the original parallel optical pulse signals are restored (decoded). This method can be used together with existing WDM or DWDM transmission systems.
In transmission by OCDM, optical pulse signals for numerous channels can be transmitted simultaneously at the same wavelength. Further, transmission methods using OCDM are methods in which the same code is used on the transmitting side and on the receiving side as a key (the code set in the encoder and decoder is sometimes called a key), so that there is the advantage of highly secure transmission.
Further, in OCDM transmission methods, compared with transmission methods using conventional optical time division multiplexing (OTDM) and WDM, simpler component elements can be used to configure devices to construct complex but flexible networks.
As means of OCDM encoding, the OCDM phase encoding method, using the optical phase as the code, is known. Specifically, Superstructured Fiber Bragg Gratings (SSFBGs) are used as the encoder and decoder. As explained above, in OCDM transmission the same code is used as a key on the transmitting side and on the receiving side; but the Bragg reflection characteristics (hereafter also called “operating characteristics”) which are the operating characteristics of the SSFBG comprised by the encoder or decoder change with the ambient temperature and other conditions. Further, when installing an SSFBG in an encoder or decoder, it is in actuality difficult to perform installation such that the operating characteristics of the SSFBGs comprised by the encoder and decoder are the same.
Hence it is necessary to adjust the operating characteristics of at least one of the SSFBGs comprised by the encoder and decoder as appropriate, such that the operating characteristics of the SSFBG comprised by the transmitting-side encoder and the SSFBG comprised by the receiving-side decoder are always the same. Constantly maintaining the operating characteristics of the SSFBG comprised by the transmitting-side encoder and the SSFBG comprised by the receiving-side decoder so as to be the same is sometimes called coordinating the operating characteristics.
In transmission by phase encoding OCDM, if the operating wavelength of the SSFBG comprised by the transmitting-side encoder and the operating wavelength of the SSFBG comprised by the receiving-side decoder deviate by several tens of pm or more, satisfactory decoding on the receiving side is not possible. That is, adjustments must be performed as necessary such that the difference in the Bragg wavelengths of the SSFBG comprised by the transmitting-side encoder and the SSFBG comprised by the receiving-side decoder is within several tens of pm.
Phase encoding OCDM using SSFBGs in an encoder and decoder has for example been reported by P. Petropoulos (see P. Petropoulos et al, “Demonstration of a 64-chip OCDMA System Using Superstructured Fiber Gratings and Time-Gating Detection”, IEEE Photonics Technology Letters, Vol. 13, No. 11, November 2001, pp. 1239-1241), but no method has been disclosed for performing adjustments such that the operating wavelengths of the SSFBG comprised by the transmitting-side encoder and the SSFBG comprised by the receiving-side decoder are always the same.
It is possible that at the time of installation in an encoder or in a decoder, an SSFBG may be installed in the encoder or decoder such that the operating wavelength of the encoder or decoder is in a shifted state. Further, during transmission or reception operation, an encoder or decoder may be affected by the ambient temperature or other changes in the environment, so that the operating wavelength changes.
Hence an object of the present invention is to provide a method to continuously coordinate the operating wavelengths of both even in such cases, as well as to provide a device which realizes such a method.